Abstract

Changes in color appearance with retinal illuminance were studied by scaling the achromatic, yellow, and blue sensation components for test lights with color temperatures from 3041 to 8650 K at 4.10, 2.18, and 0.33 log Td. At 4.10 log Td two observers showed similar pure whites (4823 and 5258 K) and narrow transition zones (1502 and 969 K) from yellow to blue chromatic response categories. The relative amounts of yellow, blue, and white varied with color temperature in a similar manner for both observers. One observer maintained similar absolute whites and transition zones for all illuminances. For the second observer the transition zone broadened and shifted to higher color temperatures as illuminance decreased. At color temperatures both above and below the transition zone chromatic saturation was greatest at the intermediate illuminance. The loss of saturation at 0.33 and 4.10 log Td is consistent with the observation that as the illuminance of a spectral color is raised above threshold, saturation increases to a maximum and then falls. Our findings reinforce the notion that at relatively low illuminances chromatic responses increase with increasing illuminance more rapidly than achromatic responses and that the opposite is true at high illuminances.

© 1993 Optical Society of America

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References

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  1. D. McL. Purdy, “Spectral hue as a function of intensity,” Am. J. Psychol. 43, 541–559 (1931).
    [Crossref]
  2. D. McL. Purdy, “The Bezold–Brücke phenomenon and contours for constant hue,” Am. J. Psychol. 49, 313–319 (1937).
    [Crossref]
  3. R. M. Boynton, J. Gordon, “Bezold–Brücke hue shift measured by color naming technique,”J. Opt. Soc. Am. 55, 78–86 (1965).
    [Crossref]
  4. D. McL. Purdy, “On the saturations and chromatic thresholds of the spectral colours,” Brit. J. Psychol. 21, 281–313 (1930–1931).
  5. A. Valberg, B. Lange-Malecki, T. Seim, “Colour changes as a function of luminance contrast,” Perception 20, 665–668 (1991).
    [Crossref]
  6. J. Walraven, J. S. Werner, “The invariance of unique white; a possible implication for normalizing cone action spectra,” Vision Res. 31, 2185–2193 (1991).
    [Crossref] [PubMed]
  7. L. M. Hurvich, D. Jameson, “A psychophysical study of white. I. Neutral adaptation,”J. Opt. Soc. Am. 41, 521–527 (1951).
    [Crossref] [PubMed]
  8. B. Drum, C. E. Sternheim, “Saturation of chromatic increments on intense achromatic backgrounds,” in Annual Meeting, Vol. 15 of 1990 OSA Technical Digest (Optical Society of America, Washington, D.C., 1990), pp. 183–184.
  9. G. Wyszecki, W. S. Stiles, Color Science (Wiley, New York, 1982) pp. 224–229.
  10. B. Drum, “Color scaling of chromatic increments on achromatic backgrounds: implications for hue signals from individual classes of cones,” Color Res. Appl. 14, 293–308 (1989).
    [Crossref]
  11. We use the term achromatic to refer to the experienced visual quality that was neither yellow nor blue (Refs. 6 and 7). At 2.18 and 4.10 log Td the achromatic sensation component was white, whereas at 0.33 log Td the field appeared darker and was more readily described as gray (see also Ref. 13). Although none of our test lights looked noticeably reddish or greenish, it is possible that a red–green discrimination could have been made if these response categories had been included in our color-naming procedure.
  12. Computations involving color temperature usually employ the reciprocal megakelvin (MK−1= 106K−1) scale, also formerly known as the microreciprocal degree, or mired, scale. Reciprocal megakelvin values are convenient to use for the conversion of a light source, since the MK−1shift values of conversion filters are additive. Also, equal MK−1shifts produce approximately equal perceptual color differences regardless of color temperature, making the MK−1scale approximately linear with color appearance. (See Ref. 9.) In Figs. 1 and 2 and Figs. 4–6, stimuli have been spaced on a reciprocal mega-kelvin scale. A corresponding color-temperature scale is given above each graph, and, to be consistent with past studies, we continue to refer to color temperature in the text.
  13. The assumption that the midpoint of the transition zone is the best white presumes symmetrical yellow and blue response functions. Although published studies [e.g., J. Larimer, D. H. Krantz, C. M. Cicerone, “Opponent process additivity—II. Yellow/blue equilibria and nonlinear models,” Vision Res. 15, 723–731, (1975)] have reported nonlinearities in the yellow–blue response functions, we would not expect such nonlinearities to change our analysis significantly.
    [Crossref] [PubMed]
  14. K. Fuld, T. A. Otto, “Colors of monochromatic lights that vary in contrast-induced brightness,” J. Opt. Soc. Am. A 2, 76–83 (1985).
    [Crossref] [PubMed]
  15. Y. Ejima, S. Takahashi, M. Akita, “Achromatic sensation for trichromatic mixture as a function of stimulus intensity,” Vision Res. 26, 1065–1071 (1986).
    [Crossref]
  16. It is possible that rod–cone interactions influence the color appearance of our stimuli at low illuminances [U. Stabell, B. Stabell, “Facilitation of chromatic cone activity by rod activity. II. Variation of chromatic-related cone activity,” Scand. J. Psychol. 12, 168–174 (1971); B. Drum, “Forced-choice colour discrimination in the dark-adapted parafovea,” in Color Vision Deficiencies V, G. Verriest, ed. (Hilger, Bristol, UK, 1980), Chap. 8, pp. 365–370] and that the shift of the transition point for CES from 5258 K at 4.10 log Td to 6443 K at 0.33 log Td is due to the activity from a relatively high number of parafoveal rods for this observer. We have found that CES is more than 1 log unit more sensitive than BD to 430-nm and 490-nm 2° test lights following 30 min of dark adaptation. These stimuli appear much less saturated to CES than to BD above threshold (see Ref. 8).
    [Crossref] [PubMed]
  17. A. Valberg, “A method for the precise determination of achromatic colours including white,” Vision Res. 11, 157–160 (1971).
    [Crossref] [PubMed]
  18. I. A. Haupt, “The selectiveness to the eye’s response to wavelength and its change with change in intensity,”J. Exp. Psychol. 5, 347–379 (1922).
    [Crossref]
  19. C. H. Graham, Y. Hsia, “Saturation and the foveal achromatic interval,”J. Opt. Soc. Am. 59, 993–997 (1969).
    [PubMed]
  20. Valberg et al. (Ref. 5) propose that the achromatic interval is probably determined by the relative sensitivities of the non-opponent M cells and the opponent P cells in the retina and in the lateral geniculate nucleus. They explain the change in hue (Bezold–Brücke effect) and saturation as a function of illuminance above the achromatic interval on the basis of the response of only opponent P cells. Our explanation of the change of saturation as a function of illuminance is consistent with this view.
  21. L. M. Hurvich, D. Jameson, “A psychophysical study of white. III. Adaptation as a variant,”J. Opt. Soc. Am. 41, 787–801 (1951).
    [Crossref] [PubMed]
  22. D. Jameson, L. M. Hurvich, “A psychophysical study of white. II. Neutral adaptation. Area and duration as variants,”J. Opt. Soc. Am. 41, 528–536 (1951).
    [Crossref] [PubMed]
  23. R. L. P. Vimal, J. Pokorny, V. C. Smith, “Appearance of steadily viewed lights,” Vision Res. 27, 1309–1318 (1987).
    [Crossref] [PubMed]
  24. W. A. H. Rushton, “Visual pigments in man,” in Handbook of Sensory Physiology, Vol. VII/1, H. J. A. Dartnall, ed. (Springer-Verlag, New York, 1972), Chap. 9, pp. 364–394.
    [Crossref]

1991 (2)

A. Valberg, B. Lange-Malecki, T. Seim, “Colour changes as a function of luminance contrast,” Perception 20, 665–668 (1991).
[Crossref]

J. Walraven, J. S. Werner, “The invariance of unique white; a possible implication for normalizing cone action spectra,” Vision Res. 31, 2185–2193 (1991).
[Crossref] [PubMed]

1989 (1)

B. Drum, “Color scaling of chromatic increments on achromatic backgrounds: implications for hue signals from individual classes of cones,” Color Res. Appl. 14, 293–308 (1989).
[Crossref]

1987 (1)

R. L. P. Vimal, J. Pokorny, V. C. Smith, “Appearance of steadily viewed lights,” Vision Res. 27, 1309–1318 (1987).
[Crossref] [PubMed]

1986 (1)

Y. Ejima, S. Takahashi, M. Akita, “Achromatic sensation for trichromatic mixture as a function of stimulus intensity,” Vision Res. 26, 1065–1071 (1986).
[Crossref]

1985 (1)

1975 (1)

The assumption that the midpoint of the transition zone is the best white presumes symmetrical yellow and blue response functions. Although published studies [e.g., J. Larimer, D. H. Krantz, C. M. Cicerone, “Opponent process additivity—II. Yellow/blue equilibria and nonlinear models,” Vision Res. 15, 723–731, (1975)] have reported nonlinearities in the yellow–blue response functions, we would not expect such nonlinearities to change our analysis significantly.
[Crossref] [PubMed]

1971 (2)

It is possible that rod–cone interactions influence the color appearance of our stimuli at low illuminances [U. Stabell, B. Stabell, “Facilitation of chromatic cone activity by rod activity. II. Variation of chromatic-related cone activity,” Scand. J. Psychol. 12, 168–174 (1971); B. Drum, “Forced-choice colour discrimination in the dark-adapted parafovea,” in Color Vision Deficiencies V, G. Verriest, ed. (Hilger, Bristol, UK, 1980), Chap. 8, pp. 365–370] and that the shift of the transition point for CES from 5258 K at 4.10 log Td to 6443 K at 0.33 log Td is due to the activity from a relatively high number of parafoveal rods for this observer. We have found that CES is more than 1 log unit more sensitive than BD to 430-nm and 490-nm 2° test lights following 30 min of dark adaptation. These stimuli appear much less saturated to CES than to BD above threshold (see Ref. 8).
[Crossref] [PubMed]

A. Valberg, “A method for the precise determination of achromatic colours including white,” Vision Res. 11, 157–160 (1971).
[Crossref] [PubMed]

1969 (1)

1965 (1)

1951 (3)

1937 (1)

D. McL. Purdy, “The Bezold–Brücke phenomenon and contours for constant hue,” Am. J. Psychol. 49, 313–319 (1937).
[Crossref]

1931 (1)

D. McL. Purdy, “Spectral hue as a function of intensity,” Am. J. Psychol. 43, 541–559 (1931).
[Crossref]

1922 (1)

I. A. Haupt, “The selectiveness to the eye’s response to wavelength and its change with change in intensity,”J. Exp. Psychol. 5, 347–379 (1922).
[Crossref]

Akita, M.

Y. Ejima, S. Takahashi, M. Akita, “Achromatic sensation for trichromatic mixture as a function of stimulus intensity,” Vision Res. 26, 1065–1071 (1986).
[Crossref]

Boynton, R. M.

Cicerone, C. M.

The assumption that the midpoint of the transition zone is the best white presumes symmetrical yellow and blue response functions. Although published studies [e.g., J. Larimer, D. H. Krantz, C. M. Cicerone, “Opponent process additivity—II. Yellow/blue equilibria and nonlinear models,” Vision Res. 15, 723–731, (1975)] have reported nonlinearities in the yellow–blue response functions, we would not expect such nonlinearities to change our analysis significantly.
[Crossref] [PubMed]

Drum, B.

B. Drum, “Color scaling of chromatic increments on achromatic backgrounds: implications for hue signals from individual classes of cones,” Color Res. Appl. 14, 293–308 (1989).
[Crossref]

B. Drum, C. E. Sternheim, “Saturation of chromatic increments on intense achromatic backgrounds,” in Annual Meeting, Vol. 15 of 1990 OSA Technical Digest (Optical Society of America, Washington, D.C., 1990), pp. 183–184.

Ejima, Y.

Y. Ejima, S. Takahashi, M. Akita, “Achromatic sensation for trichromatic mixture as a function of stimulus intensity,” Vision Res. 26, 1065–1071 (1986).
[Crossref]

Fuld, K.

Gordon, J.

Graham, C. H.

Haupt, I. A.

I. A. Haupt, “The selectiveness to the eye’s response to wavelength and its change with change in intensity,”J. Exp. Psychol. 5, 347–379 (1922).
[Crossref]

Hsia, Y.

Hurvich, L. M.

Jameson, D.

Krantz, D. H.

The assumption that the midpoint of the transition zone is the best white presumes symmetrical yellow and blue response functions. Although published studies [e.g., J. Larimer, D. H. Krantz, C. M. Cicerone, “Opponent process additivity—II. Yellow/blue equilibria and nonlinear models,” Vision Res. 15, 723–731, (1975)] have reported nonlinearities in the yellow–blue response functions, we would not expect such nonlinearities to change our analysis significantly.
[Crossref] [PubMed]

Lange-Malecki, B.

A. Valberg, B. Lange-Malecki, T. Seim, “Colour changes as a function of luminance contrast,” Perception 20, 665–668 (1991).
[Crossref]

Larimer, J.

The assumption that the midpoint of the transition zone is the best white presumes symmetrical yellow and blue response functions. Although published studies [e.g., J. Larimer, D. H. Krantz, C. M. Cicerone, “Opponent process additivity—II. Yellow/blue equilibria and nonlinear models,” Vision Res. 15, 723–731, (1975)] have reported nonlinearities in the yellow–blue response functions, we would not expect such nonlinearities to change our analysis significantly.
[Crossref] [PubMed]

Otto, T. A.

Pokorny, J.

R. L. P. Vimal, J. Pokorny, V. C. Smith, “Appearance of steadily viewed lights,” Vision Res. 27, 1309–1318 (1987).
[Crossref] [PubMed]

Purdy, D. McL.

D. McL. Purdy, “The Bezold–Brücke phenomenon and contours for constant hue,” Am. J. Psychol. 49, 313–319 (1937).
[Crossref]

D. McL. Purdy, “Spectral hue as a function of intensity,” Am. J. Psychol. 43, 541–559 (1931).
[Crossref]

D. McL. Purdy, “On the saturations and chromatic thresholds of the spectral colours,” Brit. J. Psychol. 21, 281–313 (1930–1931).

Rushton, W. A. H.

W. A. H. Rushton, “Visual pigments in man,” in Handbook of Sensory Physiology, Vol. VII/1, H. J. A. Dartnall, ed. (Springer-Verlag, New York, 1972), Chap. 9, pp. 364–394.
[Crossref]

Seim, T.

A. Valberg, B. Lange-Malecki, T. Seim, “Colour changes as a function of luminance contrast,” Perception 20, 665–668 (1991).
[Crossref]

Smith, V. C.

R. L. P. Vimal, J. Pokorny, V. C. Smith, “Appearance of steadily viewed lights,” Vision Res. 27, 1309–1318 (1987).
[Crossref] [PubMed]

Stabell, B.

It is possible that rod–cone interactions influence the color appearance of our stimuli at low illuminances [U. Stabell, B. Stabell, “Facilitation of chromatic cone activity by rod activity. II. Variation of chromatic-related cone activity,” Scand. J. Psychol. 12, 168–174 (1971); B. Drum, “Forced-choice colour discrimination in the dark-adapted parafovea,” in Color Vision Deficiencies V, G. Verriest, ed. (Hilger, Bristol, UK, 1980), Chap. 8, pp. 365–370] and that the shift of the transition point for CES from 5258 K at 4.10 log Td to 6443 K at 0.33 log Td is due to the activity from a relatively high number of parafoveal rods for this observer. We have found that CES is more than 1 log unit more sensitive than BD to 430-nm and 490-nm 2° test lights following 30 min of dark adaptation. These stimuli appear much less saturated to CES than to BD above threshold (see Ref. 8).
[Crossref] [PubMed]

Stabell, U.

It is possible that rod–cone interactions influence the color appearance of our stimuli at low illuminances [U. Stabell, B. Stabell, “Facilitation of chromatic cone activity by rod activity. II. Variation of chromatic-related cone activity,” Scand. J. Psychol. 12, 168–174 (1971); B. Drum, “Forced-choice colour discrimination in the dark-adapted parafovea,” in Color Vision Deficiencies V, G. Verriest, ed. (Hilger, Bristol, UK, 1980), Chap. 8, pp. 365–370] and that the shift of the transition point for CES from 5258 K at 4.10 log Td to 6443 K at 0.33 log Td is due to the activity from a relatively high number of parafoveal rods for this observer. We have found that CES is more than 1 log unit more sensitive than BD to 430-nm and 490-nm 2° test lights following 30 min of dark adaptation. These stimuli appear much less saturated to CES than to BD above threshold (see Ref. 8).
[Crossref] [PubMed]

Sternheim, C. E.

B. Drum, C. E. Sternheim, “Saturation of chromatic increments on intense achromatic backgrounds,” in Annual Meeting, Vol. 15 of 1990 OSA Technical Digest (Optical Society of America, Washington, D.C., 1990), pp. 183–184.

Stiles, W. S.

G. Wyszecki, W. S. Stiles, Color Science (Wiley, New York, 1982) pp. 224–229.

Takahashi, S.

Y. Ejima, S. Takahashi, M. Akita, “Achromatic sensation for trichromatic mixture as a function of stimulus intensity,” Vision Res. 26, 1065–1071 (1986).
[Crossref]

Valberg, A.

A. Valberg, B. Lange-Malecki, T. Seim, “Colour changes as a function of luminance contrast,” Perception 20, 665–668 (1991).
[Crossref]

A. Valberg, “A method for the precise determination of achromatic colours including white,” Vision Res. 11, 157–160 (1971).
[Crossref] [PubMed]

Vimal, R. L. P.

R. L. P. Vimal, J. Pokorny, V. C. Smith, “Appearance of steadily viewed lights,” Vision Res. 27, 1309–1318 (1987).
[Crossref] [PubMed]

Walraven, J.

J. Walraven, J. S. Werner, “The invariance of unique white; a possible implication for normalizing cone action spectra,” Vision Res. 31, 2185–2193 (1991).
[Crossref] [PubMed]

Werner, J. S.

J. Walraven, J. S. Werner, “The invariance of unique white; a possible implication for normalizing cone action spectra,” Vision Res. 31, 2185–2193 (1991).
[Crossref] [PubMed]

Wyszecki, G.

G. Wyszecki, W. S. Stiles, Color Science (Wiley, New York, 1982) pp. 224–229.

Am. J. Psychol. (2)

D. McL. Purdy, “Spectral hue as a function of intensity,” Am. J. Psychol. 43, 541–559 (1931).
[Crossref]

D. McL. Purdy, “The Bezold–Brücke phenomenon and contours for constant hue,” Am. J. Psychol. 49, 313–319 (1937).
[Crossref]

Brit. J. Psychol. (1)

D. McL. Purdy, “On the saturations and chromatic thresholds of the spectral colours,” Brit. J. Psychol. 21, 281–313 (1930–1931).

Color Res. Appl. (1)

B. Drum, “Color scaling of chromatic increments on achromatic backgrounds: implications for hue signals from individual classes of cones,” Color Res. Appl. 14, 293–308 (1989).
[Crossref]

J. Exp. Psychol. (1)

I. A. Haupt, “The selectiveness to the eye’s response to wavelength and its change with change in intensity,”J. Exp. Psychol. 5, 347–379 (1922).
[Crossref]

J. Opt. Soc. Am. (5)

J. Opt. Soc. Am. A (1)

Perception (1)

A. Valberg, B. Lange-Malecki, T. Seim, “Colour changes as a function of luminance contrast,” Perception 20, 665–668 (1991).
[Crossref]

Scand. J. Psychol. (1)

It is possible that rod–cone interactions influence the color appearance of our stimuli at low illuminances [U. Stabell, B. Stabell, “Facilitation of chromatic cone activity by rod activity. II. Variation of chromatic-related cone activity,” Scand. J. Psychol. 12, 168–174 (1971); B. Drum, “Forced-choice colour discrimination in the dark-adapted parafovea,” in Color Vision Deficiencies V, G. Verriest, ed. (Hilger, Bristol, UK, 1980), Chap. 8, pp. 365–370] and that the shift of the transition point for CES from 5258 K at 4.10 log Td to 6443 K at 0.33 log Td is due to the activity from a relatively high number of parafoveal rods for this observer. We have found that CES is more than 1 log unit more sensitive than BD to 430-nm and 490-nm 2° test lights following 30 min of dark adaptation. These stimuli appear much less saturated to CES than to BD above threshold (see Ref. 8).
[Crossref] [PubMed]

Vision Res. (5)

A. Valberg, “A method for the precise determination of achromatic colours including white,” Vision Res. 11, 157–160 (1971).
[Crossref] [PubMed]

Y. Ejima, S. Takahashi, M. Akita, “Achromatic sensation for trichromatic mixture as a function of stimulus intensity,” Vision Res. 26, 1065–1071 (1986).
[Crossref]

R. L. P. Vimal, J. Pokorny, V. C. Smith, “Appearance of steadily viewed lights,” Vision Res. 27, 1309–1318 (1987).
[Crossref] [PubMed]

J. Walraven, J. S. Werner, “The invariance of unique white; a possible implication for normalizing cone action spectra,” Vision Res. 31, 2185–2193 (1991).
[Crossref] [PubMed]

The assumption that the midpoint of the transition zone is the best white presumes symmetrical yellow and blue response functions. Although published studies [e.g., J. Larimer, D. H. Krantz, C. M. Cicerone, “Opponent process additivity—II. Yellow/blue equilibria and nonlinear models,” Vision Res. 15, 723–731, (1975)] have reported nonlinearities in the yellow–blue response functions, we would not expect such nonlinearities to change our analysis significantly.
[Crossref] [PubMed]

Other (6)

Valberg et al. (Ref. 5) propose that the achromatic interval is probably determined by the relative sensitivities of the non-opponent M cells and the opponent P cells in the retina and in the lateral geniculate nucleus. They explain the change in hue (Bezold–Brücke effect) and saturation as a function of illuminance above the achromatic interval on the basis of the response of only opponent P cells. Our explanation of the change of saturation as a function of illuminance is consistent with this view.

B. Drum, C. E. Sternheim, “Saturation of chromatic increments on intense achromatic backgrounds,” in Annual Meeting, Vol. 15 of 1990 OSA Technical Digest (Optical Society of America, Washington, D.C., 1990), pp. 183–184.

G. Wyszecki, W. S. Stiles, Color Science (Wiley, New York, 1982) pp. 224–229.

We use the term achromatic to refer to the experienced visual quality that was neither yellow nor blue (Refs. 6 and 7). At 2.18 and 4.10 log Td the achromatic sensation component was white, whereas at 0.33 log Td the field appeared darker and was more readily described as gray (see also Ref. 13). Although none of our test lights looked noticeably reddish or greenish, it is possible that a red–green discrimination could have been made if these response categories had been included in our color-naming procedure.

Computations involving color temperature usually employ the reciprocal megakelvin (MK−1= 106K−1) scale, also formerly known as the microreciprocal degree, or mired, scale. Reciprocal megakelvin values are convenient to use for the conversion of a light source, since the MK−1shift values of conversion filters are additive. Also, equal MK−1shifts produce approximately equal perceptual color differences regardless of color temperature, making the MK−1scale approximately linear with color appearance. (See Ref. 9.) In Figs. 1 and 2 and Figs. 4–6, stimuli have been spaced on a reciprocal mega-kelvin scale. A corresponding color-temperature scale is given above each graph, and, to be consistent with past studies, we continue to refer to color temperature in the text.

W. A. H. Rushton, “Visual pigments in man,” in Handbook of Sensory Physiology, Vol. VII/1, H. J. A. Dartnall, ed. (Springer-Verlag, New York, 1972), Chap. 9, pp. 364–394.
[Crossref]

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Figures (7)

Fig. 1
Fig. 1

Yellow and blue color-naming responses for one observer at 4.10 log Td collected in two independent sessions. Five responses were taken at each color temperature in each session.

Fig. 2
Fig. 2

Transition zones between yellow and blue for subjects BD and CES at 0.33, 2.18, and 4.10 log Td. Each transition zone is based on the average of color-naming responses collected in two sessions.

Fig. 3
Fig. 3

CIE chromaticity coordinates of transition points for subjects BD and CES at 0.33, 2.18, and 4.10 log Td in comparison with the blackbody locus.

Fig. 4
Fig. 4

Hue estimates at 0.33 log Td for subjects BD (top) and CES (bottom). Yellow, blue, and gray estimates are averages from two sessions. Error bars are standard deviations.

Fig. 5
Fig. 5

Hue estimates at 2.18 log Td for subjects BD and CES. Yellow, blue, and white estimates are averages from two sessions. Error bars are standard deviations.

Fig. 6
Fig. 6

Hue estimates at 4.10 log Td for subjeccts BD and CES. Yellow, blue, and white estimates are averages from two sessions. Error bars are standard deviations.

Fig. 7
Fig. 7

Yellow and blue estimates for 3041 and 8650 K at 0.33, 2.18, and 4.10 log Td for subjects BD and CES. Averages from two sessions are given with standard deviations as error bars.

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